The antibody and T cell proliferative responses to Staphylococcal nuclease, a small peptide antigen, have been shown to be under Ir gene control. In addition, anti-idiotypic antisera have been produced with specificity for anti-nuclease antibodies of a number of mouse strains. It has been demonstrated that these idiotypic markers are linked to allotype markers but not to MHC encoded (H-2) determinants. The present study has investigated the cellular expression of idiotypic markers in this Ir gene controlled response. In vitro augmented primary responses to TNP-nuclease could be generated after priming with either nuclease or anti-idiotypic antisera. These responses were shown to be inhibited by anti-idiotype present in culture. It was demonstrated that either antigen or anti-idiotype primed T cells were required and that treatment of the primed T cells with anti-idiotype and complement abrogated their helper cell activity. Thus, direct evidence for helper T cell expression of idiotypic markers has been obtained although as yet it is unclear whether the T cell synthesizes or acquires these cell surface determinants.